The seconds counting is not really accurate in PC-world and heavy depends on your hardware and CPU usage.

So, you post a good example of starting a task in exact second. If you need to start a task say every count of seconds you can use an endless loop. You can find an example of this technique in nnCron FAQ.

ok Thank you for reassuring me.
although I am surprised to hear that PC hardware is unable to accurately execute in a 1-second timeframe, especially considering that the PAUSE function is measured in milliseconds.

i can only think of 1 way, maybe Valery or Nicholas will have a better way. These pair of tasks should work together so that useful_task runs every 30 seconds......... also If I remember correctly from the documentation, the "caller" task must come AFTER the "callee" task.